1. Field of the Invention
This invention relates to a thermally transferable protective sheet.
2. Description of the Related Art
Conventionally, images such as tone images and mono-tone images including characters and symbols have been formed on a substrate by using a thermal transfer system. With respect to the thermal transfer system, a heat-sensitive sublimation transfer system and a heat-sensitive melt transfer system have been widely used.
Of these system, in the heat-sensitive sublimation transfer system, a thermal transfer sheet, formed by placing a dye layer made by melting or dispersing a sublimable dye serving as a colorant in a binder resin on a substrate, is used, and with this thermal transfer sheet being superposed on an image receiving sheet, energy in accordance with image information is applied to a heating device such as a thermal head so that the sublimable dye contained in the dye layer on the thermal transfer film is transferred to the image-receiving sheet; thus, an image is formed thereon. This heat-sensitive sublimation transfer system makes it possible to control the amount of dye transfer on a dot basis in response to the quantity of energy to be applied to the thermal transfer sheet; therefore, this system has the advantages of forming a superior tone image and of easily forming characters, symbols and the like.
With respect to the tone image and mono-tone image formed by the heat-sensitive sublimation transfer system, since the transferred dye is present on the surface of the transferred subject, and since the dye is not a pigment but a substance having a comparatively low molecular weight, those images are inferior in light resistance, particularly, in light resistance to ultraviolet rays. For this reason, various attempts have been made so as to improve the light resistant property. For example, a technique has been proposed in which a protective layer containing an ultraviolet-ray absorbing agent is transferred onto a transferred subject (image-receiving sheet) on which an image is formed so as to improve the light resistant property of the image (for example, Patent Document 1).
It has been conventionally known that the protective layer of this type is generally applied and formed with an amount of coat in the range from 0.1 g/m2 to 5 g/m2 (dried state). As long as the functions of the protective layer are maintained, the protective layer is preferably made as thin as possible from the viewpoint of saving resources.
However, as the layer thickness of the protective layer is made as thin as possible, rainbow unevenness due to visible light reflection comes to appear on the image surface caused by even a slight difference in layer thickness. The rainbow unevenness refers to interference fringes that occur between the image surface and the protective layer. The rainbow unevenness is remarkably observed, in particular, when a protective layer is formed on a black solid image, and since the rainbow unevenness makes it difficult to view an image printed on the transferred subject, the occurrence of this phenomenon needs to be prevented.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2000-71626